[CarPsyco84]

Its probably not your O2 sensor then, its probably really running that lean. Alot of things will make a car run too rich, too lean is somewhat rare, I would suspect your MAF first, what kind of Hz does it read when your making boost? Should be well over 1000hz by full boost.

edit: could also be a fuel delivery problem like clogged fuel filter or weak pump. Being that you said you double checked the MAF. Or check the fuel pressure regulator make sure its not leaking through the vacuum line and that its plugged in.